package display

import (
	"context"
	"encoding/binary"
	"fmt"
	"log"
	"os/exec"
	"strings"
	"time"
	"unicode/utf16"

	"github.com/BurntSushi/xgb"
	"github.com/BurntSushi/xgb/shape"
	"github.com/BurntSushi/xgb/xproto"
)

const (
	// iso10646FontPattern selects a Unicode-capable fixed font so the exact
	// fallback text keeps its diacritics on screen.
	iso10646FontPattern = "*fixed*iso10646-1"
	// asciiFontName is the degradation path when no Unicode font exists;
	// text drawn with it is folded to ASCII.
	asciiFontName = "fixed"
	// maxImageText16Units bounds one ImageText16 request to the protocol's
	// 1-byte string length (2 bytes per unit).
	maxImageText16Units = 127
	// windowTypePropertyName is the EWMH property naming a window's role.
	windowTypePropertyName = "_NET_WM_WINDOW_TYPE"
	// windowStatePropertyName is the EWMH property carrying a window's
	// initial states; a compliant window manager must honor it when the
	// window is mapped.
	windowStatePropertyName = "_NET_WM_STATE"
	// layerMarkerPropertyName tags Ground Station layer windows so sibling
	// layers — and their window-manager frames — never count as video
	// output for each other.
	layerMarkerPropertyName = "_GROUNDSTATION_LAYER"
)

// osdEventMask selects only repaint bookkeeping for the OSD overlay: no
// pointer, key or motion events, so the server never routes device input to
// the overlay even before its empty input shape is applied.
const osdEventMask = xproto.EventMaskExposure

// deviceInputEventMask are the pointer and key selection bits a layer above
// video must never select: device input belongs to the windows below.
const deviceInputEventMask = xproto.EventMaskKeyPress | xproto.EventMaskKeyRelease |
	xproto.EventMaskButtonPress | xproto.EventMaskButtonRelease |
	xproto.EventMaskPointerMotion

// LayerRole orders a Ground Station window against presented video output.
// The fallback/menu canvas stays below video; the OSD overlay stays above.
type LayerRole int

const (
	// RoleBelow marks the fullscreen fallback/menu canvas under video.
	RoleBelow LayerRole = iota
	// RoleAbove marks the transparent, input-transparent OSD overlay above
	// video.
	RoleAbove
)

// layerMarkerValue is the layer marker payload identifying a role's window
// as our own.
func layerMarkerValue(role LayerRole) string {
	if role == RoleAbove {
		return "osd"
	}
	return "kiosk"
}

// markerIsGroundstation reports whether a layer marker payload marks one of
// our own windows: those are Ground Station layers, never video output.
func markerIsGroundstation(marker string) bool {
	return marker == layerMarkerValue(RoleBelow) || marker == layerMarkerValue(RoleAbove)
}

// ewmhWindowTypeName is the EWMH window role announced for a layer. The OSD
// overlay announces DOCK, the always-above helper role, so window managers
// never decorate or bury it; the fallback canvas keeps announcing nothing,
// as before.
func ewmhWindowTypeName(role LayerRole) string {
	if role == RoleAbove {
		return "_NET_WM_WINDOW_TYPE_DOCK"
	}
	return ""
}

// fullscreenState is the wmctrl stacking state for a layer role.
func fullscreenState(role LayerRole) string {
	if role == RoleAbove {
		return "above"
	}
	return "below"
}

// fullscreenStateAtoms are the initial EWMH states announced as a property
// before mapping: every layer is genuinely fullscreen and undecorated from
// its first frame, with the canvas additionally parking below and the OSD
// overlay above. A compliant window manager must honor pre-map states; the
// post-map wmctrl request remains as reinforcement, never as the only
// fullscreen mechanism.
func fullscreenStateAtoms(role LayerRole) []string {
	atoms := []string{"_NET_WM_STATE_FULLSCREEN"}
	if role == RoleAbove {
		return append(atoms, "_NET_WM_STATE_ABOVE")
	}
	return append(atoms, "_NET_WM_STATE_BELOW")
}

// stackModeForVideoAppear is where a layer goes relative to the first
// presented video window: the canvas parks below it, the OSD rises above it.
func stackModeForVideoAppear(role LayerRole) uint32 {
	if role == RoleAbove {
		return xproto.StackModeAbove
	}
	return xproto.StackModeBelow
}

// stackReassert decides the OSD overlay's stacking correction on its
// refresh: above the tracked video window while video is live, and raised
// to the top while video is absent — the fallback canvas raises itself to
// the top when video ends and would otherwise bury the overlay's text under
// the fullscreen presentation indefinitely. Only the OSD overlay ever
// re-asserts; the canvas and video windows are never moved by this.
func stackReassert(role LayerRole, videoActive bool, topVideo xproto.Window, videoWindows map[xproto.Window]struct{}) (sibling xproto.Window, mode uint32, ok bool) {
	if role != RoleAbove {
		return 0, 0, false
	}
	if !videoActive {
		// No sibling: raise the overlay to the top of the stack.
		return 0, xproto.StackModeAbove, true
	}
	if topVideo == 0 {
		return 0, 0, false
	}
	if _, tracked := videoWindows[topVideo]; !tracked {
		return 0, 0, false
	}
	return topVideo, xproto.StackModeAbove, true
}

// raisesOnVideoEnd reports whether a layer raises itself when video ends:
// only the below canvas needs that to make the fallback visible again; the
// OSD overlay is already above everything.
func raisesOnVideoEnd(role LayerRole) bool {
	return role == RoleBelow
}

// transparentShapeKinds are the shape regions cleared to empty so the OSD
// overlay is transparent and passes input through: an empty input shape
// never receives pointer or key events, and an empty bounding shape presents
// nothing over the video below — with or without a compositor.
func transparentShapeKinds() []shape.Kind {
	return []shape.Kind{shape.SkInput, shape.SkBounding}
}

// ewmhNonVideoTypes are EWMH auxiliary window roles that never count as
// video output: desktop shells, panels, menus, dialogs and notifications
// must not suppress the fallback. Windows without the property — typical
// for plain video output — count as video.
var ewmhNonVideoTypes = map[string]bool{
	"DESKTOP":       true,
	"DOCK":          true,
	"TOOLBAR":       true,
	"MENU":          true,
	"UTILITY":       true,
	"SPLASH":        true,
	"DIALOG":        true,
	"NOTIFICATION":  true,
	"TOOLTIP":       true,
	"COMBO":         true,
	"DROPDOWN_MENU": true,
	"POPUP_MENU":    true,
}

// osdShapePadding widens each punched output region by this many pixels on
// every side, so glyph antialiasing artifacts at fixed-font edges never
// clip visibly.
const osdShapePadding = 2

// osdFallbackFontMetrics are the font metrics used when the font query
// fails: close to the common 6x13 fixed cell so a punched region still
// covers the glyphs. The OSD only ever draws terse ASCII telemetry.
func osdFallbackFontMetrics() (ascent, descent, cell int16) {
	return 11, 3, 6
}

// createWindowValues builds the CreateWindow value mask and values; the
// order must follow the mask bit order (back pixel, event mask, cursor) so
// the server reads each value correctly. A zero cursor means cursor setup
// failed: the cursor bit is dropped entirely so the window maps with the
// server default cursor — an always-visible main window takes priority
// over cursor hiding.
func createWindowValues(backPixel uint32, eventMask uint32, cursor xproto.Cursor) (uint32, []uint32) {
	if cursor == 0 {
		return xproto.CwBackPixel | xproto.CwEventMask, []uint32{backPixel, eventMask}
	}
	values := []uint32{backPixel, eventMask, uint32(cursor)}
	return xproto.CwBackPixel | xproto.CwEventMask | xproto.CwCursor, values
}

// cursorBitmapDepth is the protocol-required depth of a cursor's source and
// mask bitmaps. Creating them at any other depth makes CreateCursor fail
// asynchronously — which previously poisoned the window id and surfaced
// only as a downstream BadWindow.
const cursorBitmapDepth = 1

// eventDrainBatch bounds how many pending events one wait cycle drains. A
// busy server (video fullscreen repaints) can serve unrelated non-Expose
// events faster than they arrive; an unbounded drain would never reach the
// timer checks, so the OSD overlay's initially empty bounding shape would
// never be punched and its telemetry would never appear. Thirty-two events
// per 100 ms poll cycle sustains well over three thousand events per
// second — orders of magnitude above any plausible root-event rate — while
// bounding liveness and cancellation latency to a single batch.
const eventDrainBatch = 32

// blankCursorX is the checked X-request surface of the blank cursor setup,
// so the sequence can be driven by tests that script failures at exact
// operations and pin request order and resource reclamation.
type blankCursorX interface {
	CreatePixmapChecked(pid xproto.Pixmap, drawable xproto.Drawable, depth uint8, width, height uint16) error
	CreateGCChecked(cid xproto.Gcontext, drawable xproto.Drawable) error
	PolyFillRectangleChecked(drawable xproto.Drawable, gc xproto.Gcontext, rectangles []xproto.Rectangle) error
	FreeGC(cid xproto.Gcontext)
	CreateCursorChecked(cid xproto.Cursor, source, mask xproto.Pixmap) error
	FreePixmap(pid xproto.Pixmap)
}

type connCursorX struct{ conn *xgb.Conn }

func (x connCursorX) CreatePixmapChecked(pid xproto.Pixmap, drawable xproto.Drawable, depth uint8, width, height uint16) error {
	return xproto.CreatePixmapChecked(x.conn, depth, pid, drawable, width, height).Check()
}

func (x connCursorX) CreateGCChecked(cid xproto.Gcontext, drawable xproto.Drawable) error {
	return xproto.CreateGCChecked(x.conn, cid, drawable, 0, nil).Check()
}

func (x connCursorX) PolyFillRectangleChecked(drawable xproto.Drawable, gc xproto.Gcontext, rectangles []xproto.Rectangle) error {
	return xproto.PolyFillRectangleChecked(x.conn, drawable, gc, rectangles).Check()
}

func (x connCursorX) FreeGC(cid xproto.Gcontext) {
	xproto.FreeGC(x.conn, cid)
}

func (x connCursorX) CreateCursorChecked(cid xproto.Cursor, source, mask xproto.Pixmap) error {
	return xproto.CreateCursorChecked(x.conn, cid, source, mask,
		0, 0, 0, 0, 0, 0, 0, 0).Check()
}

func (x connCursorX) FreePixmap(pid xproto.Pixmap) {
	xproto.FreePixmap(x.conn, pid)
}

// blankCursor allocates the ids for the blank cursor sequence and runs it
// over the connection. On failure the caller closes the connection, which
// reclaims every partially created resource.
func blankCursor(conn *xgb.Conn, screen *xproto.ScreenInfo) (xproto.Cursor, error) {
	pixmap, _ := xproto.NewPixmapId(conn)
	mask, _ := xproto.NewPixmapId(conn)
	gc, _ := xproto.NewGcontextId(conn)
	cursor, _ := xproto.NewCursorId(conn)
	return blankCursorOver(connCursorX{conn}, xproto.Drawable(screen.Root),
		cursorBitmapDepth, pixmap, mask, gc, cursor)
}

// blankCursorOver performs the blank-cursor resource sequence over the
// checked request surface: 1-bit source and mask bitmaps, a matching-depth
// graphics context, both bitmaps cleared to zero (an all-zero mask displays
// nothing regardless of the source), the cursor created, then the bitmaps
// and context reclaimed. Every request is checked at its own operation: the
// first failure stops the sequence, names the exact operation, frees what
// was already created, and returns before any window request runs.
func blankCursorOver(x blankCursorX, root xproto.Drawable, depth uint8, pixmap, mask xproto.Pixmap, gc xproto.Gcontext, cursor xproto.Cursor) (xproto.Cursor, error) {
	if err := x.CreatePixmapChecked(pixmap, root, depth, 1, 1); err != nil {
		return 0, fmt.Errorf("create cursor source bitmap: %w", err)
	}
	if err := x.CreatePixmapChecked(mask, root, depth, 1, 1); err != nil {
		x.FreePixmap(pixmap)
		return 0, fmt.Errorf("create cursor mask bitmap: %w", err)
	}
	if err := x.CreateGCChecked(gc, xproto.Drawable(pixmap)); err != nil {
		x.FreePixmap(mask)
		x.FreePixmap(pixmap)
		return 0, fmt.Errorf("create cursor graphics context: %w", err)
	}
	unit := []xproto.Rectangle{{Width: 1, Height: 1}}
	if err := x.PolyFillRectangleChecked(xproto.Drawable(pixmap), gc, unit); err != nil {
		x.FreeGC(gc)
		x.FreePixmap(mask)
		x.FreePixmap(pixmap)
		return 0, fmt.Errorf("clear cursor source bitmap: %w", err)
	}
	if err := x.PolyFillRectangleChecked(xproto.Drawable(mask), gc, unit); err != nil {
		x.FreeGC(gc)
		x.FreePixmap(mask)
		x.FreePixmap(pixmap)
		return 0, fmt.Errorf("clear cursor mask bitmap: %w", err)
	}
	x.FreeGC(gc)
	if err := x.CreateCursorChecked(cursor, pixmap, mask); err != nil {
		x.FreePixmap(mask)
		x.FreePixmap(pixmap)
		return 0, fmt.Errorf("create blank cursor: %w", err)
	}
	x.FreePixmap(pixmap)
	x.FreePixmap(mask)
	return cursor, nil
}

// osdTextRect is the output region covering one drawn text: its glyph cell
// around the baseline, widened by the shape padding.
func osdTextRect(x, y, units int, cell, ascent, descent int16) xproto.Rectangle {
	pad := int16(osdShapePadding)
	return xproto.Rectangle{
		X:      int16(x) - pad,
		Y:      int16(y) - ascent - pad,
		Width:  uint16(units*int(cell) + 2*int(pad)),
		Height: uint16(int(ascent+descent) + 2*int(pad)),
	}
}

// X11Opener connects to an X server (selected by DISPLAY in the environment)
// and maps one fullscreen kiosk window for the layer role it is built with:
// the fallback/menu canvas below video (RoleBelow) or the transparent,
// input-transparent OSD overlay above video (RoleAbove). It is the display
// target for the Viture screen; future targets such as MIPI/touch provide
// their own Opener and templates.
type X11Opener struct {
	// PollInterval is how often X events are drained while waiting, so that
	// cancellation stays responsive. Zero selects 100 ms.
	PollInterval time.Duration
	// RefreshInterval is how often the layer redraws without an X event,
	// so template data that changes independently of the display server —
	// the OSD overlay's published snapshots — appears at the OSD's own
	// slower cadence. Zero keeps the purely event-driven redraws of the
	// fallback canvas.
	RefreshInterval time.Duration
	// Fullscreen applies the fullscreen window state. It defaults to
	// shelling out to wmctrl; tests may replace it. A failure is logged and
	// the window degrades to a plain window.
	Fullscreen func(args []string) error
	// Video is the shared decoded-video presence signal. The session feeds
	// it from the video output window's own lifecycle on the root screen;
	// templates consume it to show or hide themselves. When nil the opener
	// tracks presence in a private signal.
	Video *VideoPresence
	// Role orders this layer's window against presented video: RoleBelow
	// for the fallback/menu canvas, RoleAbove for the OSD overlay.
	Role LayerRole
}

func (o X11Opener) presence() *VideoPresence {
	if o.Video != nil {
		return o.Video
	}
	return NewVideoPresence()
}

func (o X11Opener) applyFullscreen(args []string) error {
	if o.Fullscreen != nil {
		return o.Fullscreen(args)
	}
	return exec.Command("wmctrl", args...).Run()
}

func (o X11Opener) pollInterval() time.Duration {
	if o.PollInterval > 0 {
		return o.PollInterval
	}
	return 100 * time.Millisecond
}

// livenessRequest is one liveness round trip on a connection: it resolves
// its request and reports only the error.
type livenessRequest func() error

// connLivenessProbe returns the liveness round trip used after mapping a
// window and while a session waits for events. GetInputFocus carries a
// reply, so its cookie must be resolved with Reply: calling Check on a
// reply-bearing cookie fails at runtime with the XGB reply-cookie error
// ("Cannot call 'Check' on a cookie that is expecting a *reply*"), which
// killed every layer open and reconnect on the device.
func connLivenessProbe(conn *xgb.Conn) livenessRequest {
	return func() error {
		return resolveLiveness(xproto.GetInputFocus(conn))
	}
}

// resolveLiveness consumes one issued reply-bearing liveness cookie: the
// reply is read and discarded, and only the error propagates. The cookie
// must be resolved with Reply exactly once — Check on a reply-bearing
// cookie returns the XGB reply-cookie error instead of ever reaching the
// server, so a probe resolved with Check fails on every call regardless of
// server health.
func resolveLiveness(cookie xproto.GetInputFocusCookie) error {
	_, err := cookie.Reply()
	return err
}

// confirmMapped runs the post-map liveness stage of Open: a successful
// reply proves the window is mapped and the server alive; a failure is
// wrapped as the map-kiosk-window error so the caller closes the
// connection and drops the mapped window.
func confirmMapped(probe livenessRequest) error {
	if err := probe(); err != nil {
		return fmt.Errorf("map kiosk window: %w", err)
	}
	return nil
}

// Open connects to the X server and maps the kiosk window. Failure is
// expected while the display manager is starting; the caller retries.
func (o X11Opener) Open(ctx context.Context) (Session, error) {
	conn, err := xgb.NewConn()
	if err != nil {
		return nil, fmt.Errorf("connect X11: %w", err)
	}

	screen := xproto.Setup(conn).DefaultScreen(conn)
	window, err := xproto.NewWindowId(conn)
	if err != nil {
		conn.Close()
		return nil, fmt.Errorf("allocate window: %w", err)
	}

	// The OSD overlay selects no device input even before its empty input
	// shape exists; the canvas keeps its legacy exposure-only mask.
	eventMask := uint32(xproto.EventMaskExposure)
	if o.Role == RoleAbove {
		eventMask = osdEventMask
	}
	// The blank cursor hides the pointer over this layer from its first
	// frame and stays on the layer across no-video/video transitions. It
	// is optional: a cursor setup failure is surfaced as a bounded warning
	// and the window still maps with the server default cursor — an
	// always-visible main window takes priority over cursor hiding, and a
	// cursor failure must never destroy or reconnect the display.
	cursor, cursorErr := blankCursor(conn, screen)
	if cursorErr != nil {
		log.Printf("blank cursor unavailable (%v); mapping with the default cursor", cursorErr)
		cursor = 0
	}
	valueMask, values := createWindowValues(screen.BlackPixel, eventMask, cursor)
	// The window creation is checked at its own operation: a failure here
	// is reported exactly, and no dependent request — properties, shapes,
	// map, liveness — ever runs against an invalid window.
	if err := xproto.CreateWindowChecked(conn, screen.RootDepth, window, screen.Root,
		0, 0, screen.WidthInPixels, screen.HeightInPixels, 0,
		xproto.WindowClassInputOutput, screen.RootVisual, valueMask,
		values).Check(); err != nil {
		// The window was never created: reclaim the connection (and the
		// live cursor with a best-effort free) exactly like the other
		// open failures, so reconnect attempts cannot leak either.
		if cursor != 0 {
			xproto.FreeCursor(conn, cursor)
		}
		conn.Close()
		return nil, fmt.Errorf("create kiosk window: %w", err)
	}
	// Tag the window as a Ground Station layer and announce its EWMH role
	// before mapping, so no observer or window manager ever sees an
	// untagged frame of our own layers.
	markerAtom := internAtom(conn, layerMarkerPropertyName)
	if markerAtom != 0 {
		marker := layerMarkerValue(o.Role)
		xproto.ChangeProperty(conn, xproto.PropModeReplace, window, markerAtom,
			xproto.AtomString, 8, uint32(len(marker)), []byte(marker))
	}
	if name := ewmhWindowTypeName(o.Role); name != "" {
		typeProp := internAtom(conn, windowTypePropertyName)
		roleAtom := internAtom(conn, name)
		if typeProp != 0 && roleAtom != 0 {
			value := make([]byte, 4)
			binary.LittleEndian.PutUint32(value, uint32(roleAtom))
			xproto.ChangeProperty(conn, xproto.PropModeReplace, window, typeProp,
				xproto.AtomAtom, 32, 1, value)
		}
	}
	// Announce the initial fullscreen state before mapping: a compliant
	// window manager then maps the layer fullscreen and undecorated from
	// its first frame, instead of a decorated, repositioned window that
	// only becomes fullscreen after a later wmctrl round trip (or never,
	// when wmctrl is missing).
	stateProp := internAtom(conn, windowStatePropertyName)
	if stateProp != 0 {
		var stateData []byte
		for _, name := range fullscreenStateAtoms(o.Role) {
			if atom := internAtom(conn, name); atom != 0 {
				value := make([]byte, 4)
				binary.LittleEndian.PutUint32(value, uint32(atom))
				stateData = append(stateData, value...)
			}
		}
		if len(stateData) > 0 {
			xproto.ChangeProperty(conn, xproto.PropModeReplace, window, stateProp,
				xproto.AtomAtom, 32, uint32(len(stateData)/4), stateData)
		}
	}
	// The OSD overlay must be transparent and input-transparent from its
	// first mapped frame: the shapes are emptied before mapping, and a
	// server without the shape extension never gets the overlay at all, so
	// it can never present an unshaped frame over pilot video.
	if o.Role == RoleAbove {
		if err := applyTransparentShapes(conn, window); err != nil {
			conn.Close()
			return nil, err
		}
	}
	xproto.MapWindow(conn, window)
	// Watch the root screen so video output announces itself: a video
	// window mapping is the first presented video frame, its unmap or
	// destroy is video end. WFB and launcher processes are never consulted.
	xproto.ChangeWindowAttributes(conn, screen.Root,
		xproto.CwEventMask, []uint32{xproto.EventMaskSubstructureNotify})
	// Round trip confirms the window is mapped and the server is alive;
	// the reply-bearing cookie is resolved with Reply (see
	// connLivenessProbe).
	if err := confirmMapped(connLivenessProbe(conn)); err != nil {
		conn.Close()
		return nil, err
	}

	// A layer reconnect must never cover running video: if video windows
	// are already presented, record presence and take our side relative to
	// the topmost one before anything is drawn.
	presence := o.presence()
	typeAtom := internWindowTypeAtom(conn)
	videoWindows := scanVideoWindows(conn, screen.Root, window, typeAtom, markerAtom)
	var topVideo xproto.Window
	if len(videoWindows) > 0 {
		presence.SetActive(true)
		topVideo = videoWindows[len(videoWindows)-1]
		stackWindowRelative(conn, window, topVideo,
			stackModeForVideoAppear(o.Role))
	} else {
		presence.SetActive(false)
	}

	// Fullscreen on the root screen, on our side of the video window: pilot
	// video owns rendering priority whenever it is available. The canvas
	// raises itself only after video ends, to bring the fallback back; the
	// OSD overlay is already above video. A missing wmctrl only degrades to
	// a plain window and must not kill the layer.
	if err := o.applyFullscreen(x11FullscreenArgs(uint32(window), o.Role)); err != nil {
		log.Printf("kiosk fullscreen state: %v (window stays a plain window)", err)
	}

	gc, err := xproto.NewGcontextId(conn)
	if err != nil {
		conn.Close()
		return nil, fmt.Errorf("allocate graphics context: %w", err)
	}
	font, err := xproto.NewFontId(conn)
	if err != nil {
		conn.Close()
		return nil, fmt.Errorf("allocate font: %w", err)
	}
	// Prefer a Unicode font so the exact fallback text renders with its
	// diacritics; without one, drawn text degrades to ASCII.
	fontName, unicodeFont := selectFontName(conn)
	xproto.OpenFont(conn, font, uint16(len(fontName)), fontName)
	xproto.CreateGC(conn, gc, xproto.Drawable(window),
		xproto.GcForeground|xproto.GcBackground|xproto.GcFont,
		[]uint32{screen.WhitePixel, screen.BlackPixel, uint32(font)})

	// The OSD overlay punches its output shape per drawn region, so it
	// needs the real glyph metrics; the canvas never punches shapes.
	ascent, descent, cell := osdFallbackFontMetrics()
	if o.Role == RoleAbove {
		if info, err := xproto.QueryFont(conn, xproto.Fontable(font)).Reply(); err == nil {
			ascent, descent = int16(info.FontAscent), int16(info.FontDescent)
			if width := int16(info.MaxBounds.CharacterWidth); width > 0 {
				cell = width
			}
		}
	}

	return &x11Session{
		conn:         conn,
		window:       window,
		gc:           gc,
		width:        int(screen.WidthInPixels),
		height:       int(screen.HeightInPixels),
		black:        screen.BlackPixel,
		white:        screen.WhitePixel,
		poll:         o.pollInterval(),
		refresh:      o.RefreshInterval,
		video:        presence,
		videoWindows: videoWindowSet(videoWindows),
		topVideo:     topVideo,
		typeAtom:     typeAtom,
		markerAtom:   markerAtom,
		role:         o.Role,
		unicodeFont:  unicodeFont,
		cursor:       cursor,
		fontAscent:   ascent,
		fontDescent:  descent,
		fontCell:     cell,
		pollEvent:    conn.PollForEvent,
		probe:        connLivenessProbe(conn),
	}, nil
}

func videoWindowSet(windows []xproto.Window) map[xproto.Window]struct{} {
	set := make(map[xproto.Window]struct{}, len(windows))
	for _, w := range windows {
		set[w] = struct{}{}
	}
	return set
}

func x11FullscreenArgs(window uint32, role LayerRole) []string {
	return []string{"-i", "-r", fmt.Sprintf("0x%08x", window),
		"-b", "add,fullscreen," + fullscreenState(role)}
}

// applyTransparentShapes empties the overlay's input and bounding shapes
// before it is mapped: it then never intercepts pointer or key input and
// never presents pixels over pilot video, with or without a compositor.
// Future OSD content will punch its output shape per drawn region.
func applyTransparentShapes(conn *xgb.Conn, window xproto.Window) error {
	if err := shape.Init(conn); err != nil {
		return fmt.Errorf("shape extension: %w", err)
	}
	for _, kind := range transparentShapeKinds() {
		if err := shape.RectanglesChecked(conn, shape.SoSet, kind, 0,
			window, 0, 0, nil).Check(); err != nil {
			return fmt.Errorf("empty shape region %d: %w", kind, err)
		}
	}
	return nil
}

// scanVideoWindows lists already-presented video windows on the root screen
// in bottom-to-top stacking order, so a layer reconnect can take its side
// relative to the topmost one and never cover running video.
func scanVideoWindows(conn *xgb.Conn, root, own xproto.Window, typeAtom, markerAtom xproto.Atom) []xproto.Window {
	tree, err := xproto.QueryTree(conn, root).Reply()
	if err != nil {
		return nil
	}
	var windows []xproto.Window
	for _, w := range tree.Children {
		if w != own && presentedVideoWindow(conn, w, typeAtom, markerAtom) {
			windows = append(windows, w)
		}
	}
	return windows
}

// windowFacts is the presentation evidence gathered for one candidate
// window, so the video decision stays a pure function.
type windowFacts struct {
	inputOutput      bool
	overrideRedirect bool
	viewable         bool
	auxiliary        bool
	groundstation    bool
}

// isPresentedVideoWindow decides whether a window is video output. Only a
// real, presented output window counts as video: input-only and
// override-redirect helpers such as menus or notifications never do, and
// neither do windows carrying an EWMH auxiliary window type or a Ground
// Station layer marker.
func isPresentedVideoWindow(f windowFacts) bool {
	return f.inputOutput && !f.overrideRedirect && f.viewable &&
		!f.auxiliary && !f.groundstation
}

// presentedVideoWindow gathers the presentation facts of w.
func presentedVideoWindow(conn *xgb.Conn, w xproto.Window, typeAtom, markerAtom xproto.Atom) bool {
	attrs, err := xproto.GetWindowAttributes(conn, w).Reply()
	if err != nil {
		return false
	}
	return isPresentedVideoWindow(windowFacts{
		inputOutput:      attrs.Class == xproto.WindowClassInputOutput,
		overrideRedirect: attrs.OverrideRedirect,
		viewable:         attrs.MapState == xproto.MapStateViewable,
		auxiliary:        auxiliaryWindowType(conn, w, typeAtom),
		groundstation:    groundstationLayerWindow(conn, w, markerAtom),
	})
}

// markerWalkBounds bound the descendant search for layer markers: window
// managers reparent client windows into frame windows, so a Ground Station
// layer must be recognized through its frame, and the search must stay
// cheap even on a busy desktop screen. depth must cover our window below
// one window-manager frame; nodes bounds the total windows inspected.
func markerWalkBounds() (depth, nodes int) {
	return 4, 64
}

// groundstationLayerWindow reports whether w itself or any window below it
// carries our layer marker. Window managers frame our windows, so the
// marker must be found through frame windows too: otherwise a framed
// Ground Station layer could count as video output and permanently
// suppress the fallback. When the search hits its bounds or the server
// refuses, it reports not-found, degrading to the plain window checks.
func groundstationLayerWindow(conn *xgb.Conn, w xproto.Window, markerAtom xproto.Atom) bool {
	if markerAtom == 0 {
		return false
	}
	depth, nodes := markerWalkBounds()
	visited := 0
	var walk func(xproto.Window, int) bool
	walk = func(w xproto.Window, depth int) bool {
		if visited >= nodes {
			return false
		}
		visited++
		if carriesLayerMarker(conn, w, markerAtom) {
			return true
		}
		if depth <= 0 {
			return false
		}
		tree, err := xproto.QueryTree(conn, w).Reply()
		if err != nil {
			return false
		}
		for _, child := range tree.Children {
			if walk(child, depth-1) {
				return true
			}
		}
		return false
	}
	return walk(w, depth)
}

// carriesLayerMarker reports whether w itself carries our layer marker.
func carriesLayerMarker(conn *xgb.Conn, w xproto.Window, markerAtom xproto.Atom) bool {
	reply, err := xproto.GetProperty(conn, false, w, markerAtom,
		xproto.GetPropertyTypeAny, 0, 64).Reply()
	if err != nil || reply.ValueLen == 0 {
		return false
	}
	return markerIsGroundstation(string(reply.Value))
}

// internAtom resolves an atom by name, or 0 when the server refuses,
// disabling the checks that would use it.
func internAtom(conn *xgb.Conn, name string) xproto.Atom {
	reply, err := xproto.InternAtom(conn, false,
		uint16(len(name)), name).Reply()
	if err != nil {
		return 0
	}
	return reply.Atom
}

func internWindowTypeAtom(conn *xgb.Conn) xproto.Atom {
	return internAtom(conn, windowTypePropertyName)
}

// auxiliaryWindowType reports whether the window carries an EWMH window
// type naming a non-video helper role. A missing property means the window
// counts as video.
func auxiliaryWindowType(conn *xgb.Conn, w xproto.Window, typeAtom xproto.Atom) bool {
	if typeAtom == 0 {
		return false
	}
	reply, err := xproto.GetProperty(conn, false, w, typeAtom,
		xproto.GetPropertyTypeAny, 0, 32).Reply()
	if err != nil || reply.ValueLen < 4 {
		return false
	}
	for i := 0; i+4 <= len(reply.Value); i += 4 {
		atom := xproto.Atom(binary.LittleEndian.Uint32(reply.Value[i : i+4]))
		name, err := xproto.GetAtomName(conn, atom).Reply()
		if err != nil {
			continue
		}
		if ewmhNonVideoTypes[name.Name] {
			return true
		}
	}
	return false
}

// stackWindowRelative moves window to the given side of sibling.
func stackWindowRelative(conn *xgb.Conn, window, sibling xproto.Window, mode uint32) {
	xproto.ConfigureWindow(conn, window,
		xproto.ConfigWindowSibling|xproto.ConfigWindowStackMode,
		[]uint32{uint32(sibling), mode})
}

// videoChange is the video-lifecycle meaning of one root structure event.
type videoChange int

const (
	videoUnrelated videoChange = iota
	videoAppeared
	videoDisappeared
)

// classifyVideoChange maps a root structure event onto the video lifecycle.
// own is the kiosk window. The video output windows themselves are the
// presence signal: their maps are the first presented video output and
// their unmaps or destroys are video ends, so neither WFB nor launcher
// processes are ever consulted. Whether an announced window actually is
// video is decided by the session using the presented-window checks.
func classifyVideoChange(event xgb.Event, own xproto.Window) (videoChange, xproto.Window) {
	switch e := event.(type) {
	case xproto.MapNotifyEvent:
		if e.Window == own {
			return videoUnrelated, 0
		}
		return videoAppeared, e.Window
	case xproto.UnmapNotifyEvent:
		if e.Window == own {
			return videoUnrelated, 0
		}
		return videoDisappeared, e.Window
	case xproto.DestroyNotifyEvent:
		return videoDisappeared, e.Window
	}
	return videoUnrelated, 0
}

// addVideoWindow records w as presented video output and reports whether
// presence changed (the set went from empty to non-empty).
func addVideoWindow(windows map[xproto.Window]struct{}, w xproto.Window) bool {
	if _, tracked := windows[w]; tracked {
		return false
	}
	wasEmpty := len(windows) == 0
	windows[w] = struct{}{}
	return wasEmpty
}

// removeVideoWindow drops w from the presented video windows and reports
// whether presence changed (the set became empty).
func removeVideoWindow(windows map[xproto.Window]struct{}, w xproto.Window) bool {
	if _, tracked := windows[w]; !tracked {
		return false
	}
	delete(windows, w)
	return len(windows) == 0
}

// x11Session implements both Session and Canvas for one X11 connection.
type x11Session struct {
	conn         *xgb.Conn
	window       xproto.Window
	gc           xproto.Gcontext
	width        int
	height       int
	black        uint32
	white        uint32
	poll         time.Duration
	refresh      time.Duration
	video        *VideoPresence
	videoWindows map[xproto.Window]struct{}
	// topVideo is the video window the OSD overlay must sit above; it is
	// re-asserted on the overlay's refresh because window managers may
	// promote fullscreened video back on top after the appear restack.
	topVideo    xproto.Window
	typeAtom    xproto.Atom
	markerAtom  xproto.Atom
	role        LayerRole
	unicodeFont bool
	// cursor is the blank cursor applied to the layer window at creation;
	// it is freed when the session closes.
	cursor xproto.Cursor
	// fontAscent, fontDescent and fontCell describe the drawn font so the
	// OSD overlay can punch its output shape around each text's glyph cell.
	fontAscent  int16
	fontDescent int16
	fontCell    int16
	// drawn accumulates the output regions of texts drawn since the last
	// clear; the OSD overlay's flush punches its bounding shape over it.
	drawn []xproto.Rectangle
	// pollEvent drains one pending X event; conn.PollForEvent in
	// production. Tests script it.
	pollEvent func() (xgb.Event, xgb.Error)
	// probe performs one reply-bearing liveness round trip; its error ends
	// the session. connLivenessProbe in production. Tests script it.
	probe livenessRequest
}

func (s *x11Session) Canvas() Canvas { return s }

func (s *x11Session) Size() (int, int) { return s.width, s.height }

func (s *x11Session) Clear() {
	// A fresh draw pass starts with no output regions: the OSD overlay's
	// next flush re-punches its shape over exactly what this pass draws.
	s.resetOSDRegions()
	// ClearArea tiles the window background (black, set at creation); it
	// involves no graphics context.
	xproto.ClearArea(s.conn, false, s.window, 0, 0,
		uint16(s.width), uint16(s.height))
}

// resetOSDRegions forgets the output regions of the previous draw pass.
func (s *x11Session) resetOSDRegions() {
	s.drawn = s.drawn[:0]
}

func (s *x11Session) Text(x, y int, message string) {
	xproto.ChangeGC(s.conn, s.gc, xproto.GcForeground, []uint32{s.white})
	if s.unicodeFont {
		chars := utf16Char2b(message, maxImageText16Units)
		xproto.ImageText16(s.conn, byte(len(chars)), xproto.Drawable(s.window), s.gc,
			int16(x), int16(y), chars)
		s.recordOSDText(x, y, len(chars))
		return
	}
	message = foldToASCII(message)
	if len(message) > 255 {
		message = message[:255]
	}
	xproto.ImageText8(s.conn, byte(len(message)), xproto.Drawable(s.window), s.gc,
		int16(x), int16(y), message)
	s.recordOSDText(x, y, len(message))
}

// recordOSDText remembers one drawn text's output region so the overlay's
// flush can punch its bounding shape over exactly what was drawn. Only the
// OSD overlay punches shapes; the fallback canvas always stays plain.
func (s *x11Session) recordOSDText(x, y, units int) {
	if s.role != RoleAbove || units <= 0 {
		return
	}
	s.drawn = append(s.drawn,
		osdTextRect(x, y, units, s.fontCell, s.fontAscent, s.fontDescent))
}

func (s *x11Session) Flush() {
	// The OSD overlay presents pixels only where this draw pass put text:
	// its bounding shape is re-punched over the recorded regions while its
	// input shape stays empty, so it never covers video elsewhere and
	// never intercepts input. A failure here is a broken connection; the
	// liveness probe below ends the session and the layer reconnects.
	if s.role == RoleAbove {
		if err := applyOSDShapes(s.conn, s.window, s.drawn); err != nil {
			log.Printf("osd overlay shapes: %v", err)
		}
	}
	s.conn.Sync()
}

// applyOSDShapes sets the overlay's bounding shape to the drawn regions
// (possibly none) and keeps its input shape empty.
func applyOSDShapes(conn *xgb.Conn, window xproto.Window, rects []xproto.Rectangle) error {
	if err := shape.RectanglesChecked(conn, shape.SoSet, shape.SkBounding, 0,
		window, 0, 0, rects).Check(); err != nil {
		return fmt.Errorf("bounding shape: %w", err)
	}
	if err := shape.RectanglesChecked(conn, shape.SoSet, shape.SkInput, 0,
		window, 0, 0, nil).Check(); err != nil {
		return fmt.Errorf("empty input shape: %w", err)
	}
	return nil
}

func (s *x11Session) WaitEvent(ctx context.Context) (bool, error) {
	ticker := time.NewTicker(s.poll)
	defer ticker.Stop()
	lastRefresh := time.Now()
	for {
		// Drain a bounded batch of pending events: a busy server (video
		// fullscreen repaints) can serve unrelated non-Expose events faster
		// than they arrive, and an unbounded drain would never reach the
		// timer checks — the OSD overlay's initially empty bounding shape
		// would never be punched and its telemetry would never appear.
		for drained := 0; drained < eventDrainBatch; drained++ {
			event, err := s.pollEvent()
			if err != nil {
				return false, err
			}
			if event == nil {
				break
			}
			if x11WantsRedraw(event) {
				return true, nil
			}
			if s.applyVideoChange(event) {
				return true, nil
			}
		}
		// The OSD overlay redraws on its own slower cadence than video so
		// newly published snapshots appear without waiting for X events.
		// A zero interval disables it, keeping the fallback canvas purely
		// event-driven.
		if s.refresh > 0 && time.Since(lastRefresh) >= s.refresh {
			lastRefresh = time.Now()
			// The redraw is also where the overlay re-claims its place
			// above fullscreen video: window managers may have promoted
			// the video window back on top since the appear restack.
			s.reassertStacking()
			return true, nil
		}
		// Round trip keeps the connection liveness visible even without
		// incoming events; the reply-bearing cookie is resolved with Reply
		// (see connLivenessProbe).
		if err := s.probe(); err != nil {
			return false, err
		}
		select {
		case <-ctx.Done():
			return false, ctx.Err()
		case <-ticker.C:
		}
	}
}

// reassertStacking re-claims the overlay's stacking place on the OSD's
// slower refresh cadence: above the live video window, or on top of the
// stack while video is absent so the fallback canvas can never bury it. A
// no-op for the canvas layer.
func (s *x11Session) reassertStacking() {
	videoActive := s.video != nil && s.video.Active()
	sibling, mode, ok := stackReassert(s.role, videoActive, s.topVideo, s.videoWindows)
	if !ok {
		return
	}
	if sibling != 0 {
		stackWindowRelative(s.conn, s.window, sibling, mode)
		return
	}
	xproto.ConfigureWindow(s.conn, s.window,
		xproto.ConfigWindowStackMode, []uint32{mode})
}

// applyVideoChange folds one root structure event into the shared video
// presence and restacks this layer so video keeps rendering priority: the
// canvas parks below video and the OSD overlay rises above it. It reports
// whether the template must be redrawn: video appearing makes the fallback
// disappear and video ending brings it back. Presence follows the set of
// presented video windows, so an unrelated window appearing or ending can
// never hide or resurrect a template on its own.
func (s *x11Session) applyVideoChange(event xgb.Event) bool {
	change, w := classifyVideoChange(event, s.window)
	switch change {
	case videoAppeared:
		// Menus, notifications, other auxiliary windows and our own sibling
		// layers never count as video; only a presented output window does.
		if !presentedVideoWindow(s.conn, w, s.typeAtom, s.markerAtom) {
			return false
		}
		if !addVideoWindow(s.videoWindows, w) {
			return false
		}
		// The first presented video output takes priority: park on our side
		// of it, and remember it as the restack target for later re-claims.
		s.topVideo = w
		stackWindowRelative(s.conn, s.window, w, stackModeForVideoAppear(s.role))
		return s.video.SetActive(true)
	case videoDisappeared:
		if !removeVideoWindow(s.videoWindows, w) {
			return false
		}
		if s.topVideo == w {
			s.topVideo = 0
		}
		// Only the below canvas raises itself so the fallback returns
		// visibly even if some other window was stacked above it; the OSD
		// overlay is already above video.
		if raisesOnVideoEnd(s.role) {
			xproto.ConfigureWindow(s.conn, s.window,
				xproto.ConfigWindowStackMode, []uint32{xproto.StackModeAbove})
		}
		return s.video.SetActive(false)
	default:
		return false
	}
}

func x11WantsRedraw(event xgb.Event) bool {
	switch event.(type) {
	case xproto.ExposeEvent:
		return true
	default:
		return false
	}
}

// selectFontName resolves a Unicode-capable font for the fallback text. It
// reports false when none is installed, in which case drawn text is folded
// to ASCII.
func selectFontName(conn *xgb.Conn) (string, bool) {
	reply, err := xproto.ListFontsWithInfo(conn, 1,
		uint16(len(iso10646FontPattern)), iso10646FontPattern).Reply()
	if err != nil || reply.NameLen == 0 {
		return asciiFontName, false
	}
	return reply.Name, true
}

// utf16Char2b encodes the message as UTF-16 units for ImageText16 so the
// exact fallback text keeps its diacritics, truncating to maxUnits because
// the request carries a 1-byte string length. A cut never splits a
// surrogate pair.
func utf16Char2b(message string, maxUnits int) []xproto.Char2b {
	if maxUnits <= 0 {
		return nil
	}
	units := utf16.Encode([]rune(message))
	if len(units) > maxUnits {
		units = units[:maxUnits]
		if last := units[len(units)-1]; last >= 0xD800 && last <= 0xDBFF {
			units = units[:len(units)-1]
		}
	}
	chars := make([]xproto.Char2b, len(units))
	for i, unit := range units {
		chars[i] = xproto.Char2b{Byte1: byte(unit >> 8), Byte2: byte(unit)}
	}
	return chars
}

// latinFolds maps the Polish diacritics to plain ASCII for fonts without
// Unicode glyphs.
var latinFolds = map[rune]byte{
	'ą': 'a', 'ć': 'c', 'ę': 'e', 'ł': 'l', 'ń': 'n', 'ó': 'o', 'ś': 's', 'ź': 'z', 'ż': 'z',
	'Ą': 'A', 'Ć': 'C', 'Ę': 'E', 'Ł': 'L', 'Ń': 'N', 'Ó': 'O', 'Ś': 'S', 'Ź': 'Z', 'Ż': 'Z',
}

// foldToASCII degrades text for legacy fonts: Polish diacritics fold to
// their base letters and other non-ASCII runes are dropped.
func foldToASCII(message string) string {
	ascii := true
	for _, r := range message {
		if r >= 0x80 {
			ascii = false
			break
		}
	}
	if ascii {
		return message
	}
	var b strings.Builder
	for _, r := range message {
		if r < 0x80 {
			b.WriteRune(r)
			continue
		}
		if folded, ok := latinFolds[r]; ok {
			b.WriteByte(folded)
		}
	}
	return b.String()
}

func (s *x11Session) Close() error {
	// Free the blank cursor when one was created; closing the connection
	// then makes the X server reclaim everything else, including the kiosk
	// window.
	if s.cursor != 0 {
		xproto.FreeCursor(s.conn, s.cursor)
	}
	s.conn.Close()
	return nil
}